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measuring photosynthetic active radiation (PAR) along a 1-m probe was chosen for the light
penetration measurements. Marsh types included
Spartina alterniflora
and
Juncus roemerianus
(saline marshes),
Spartina patens
(brackish marsh), and
Panicum hemitomon
(fresh marsh). Four
variables related to field data collection and postdata analysis design were examined with respect
to fulfilling data reliability and accuracy and maximizing the potential for operational use for remote
sensing calibration and assessment of classification accuracy. These variables included: (1) the
horizontal (planar) and (2) vertical (canopy profile) spatial sampling frequencies, (3) the description
and possible exclusion of atypical canopy structures, and (4) the normalization of measurements
at different sun elevations.
Early testing showed 30-m transects in the north and south and east and west directions
combined with light penetration measurements every 3 m helped ensure more accurate depiction
of the local variability and matched or encompassed the spatial resolution of most common resource
remote sensing sensors. Similarly, vertical light attenuation profiles derived from sampling the
canopy every 20 cm from the ground surface to above the canopy improved reliability, consistency,
and completeness of repeated measurements. Accounting for the state of the canopy as undisturbed,
partial gap, or completely lodged at each profile location was found to increase the comparability
and detail of PAR attenuation profiles taken at the same site during multiple occupations and
between sites and marsh types.
In all marshes, as sun zenith increased the rate of light fall-off with canopy depth increased,
although this effect was more noticeable in
Juncus roemerianus
and less in
Spartina patens
canopies.
To remove the sun zenith influences, a method was tested to normalize canopy PAR penetration
measurements to a nadir sun zenith. The success of the removal was linked to the spherical canopy
leaf orientation used by the normalization. PAR normalizations seemed more successful when used
in more vertical canopies such as
Juncus roemerianus
and
Spartina alterniflora
, least successful
in highly lodged canopies such as
Spartina patens
, and more dependent on seasonal canopy
development in marshes such as
Panicum hemitomon
. In all marshes, application of the normal-
ization increased alignment of PAR attenuation profile taken at different sun zeniths and alignment
with the expected progression of canopy structure over time.
ACKNOWLEDGMENTS
We thank Joe White and James Burnett of the U.S. Fish and Wildlife Service for access to the
St. Marks National Wildlife Refuge, Florida, and John Fort and Doug Scott for help in field logistics
and data collections. We are grateful to U.S. Geological Survey personnel Allison Craver, Kevin
McRae, Dal Chappell, Richard Day, and Steve Laine for the many hours of work on planning field
logistics and data collections on this study and also thank Beth Vairin for editing this manuscript.
Mention of trade names or commercial products is not an endorsement or recommendation for use
by the U.S. Government.
REFERENCES
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Louisiana State University, Baton Rouge, 1970.
Decagon Devices, Sunfleck Ceptometer Reference Guide
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Decagon Devices, Inc., Pullman, WA, 1991.
Goudriaan, J., Crop Micrometeorology: A Simulation Study, Centre for Agricultural Publishing and Docu-
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